I just re-read your original post, you are using Boston mains and the Polk center. Its possible that the tonal mismatch between the mains and the center is causing the issue. In the near field (when you are up close to the center), the sound from the mains is not interfering. When you are sitting on the couch, you hear sound that is the superposition of the sound from all three ie. Main L,R and Center. That is why it is recommended to have the center tone matched to the mains. In the physical world it equates to same brand, same product line.
Also, how big is your listening area? While it is possible that the HK 125 is under powered for your needs, it may or may not be the cause of the issue.